The use of photoinitiator systems to initiate photopolymerization is well known. When irradiated by actinic radiation, the photoinitiator system generates free radicals, which initiate polymerization of the monomer or monomers. The photoinitiator system may be a single compound which absorbs actinic radiation and forms the initiating radicals, or it may consist of several different materials that undergo a complex series of reactions to produce radicals. Added components, known as coinitiators, which do not absorb actinic radiation, but which increase the efficiency of the photoinitiator system, may also be present.
Many of the well-known photoinitiator systems have limited applicability because they are activated only by ultraviolet radiation. The availability of reliable, relatively inexpensive, visible lasers, which can expose the photopolymer directly, eliminating the intermediate photomask, has made it essential that initiator systems which can be activated by visible radiation be developed. Applications for photopolymerizable compositions which are sensitive to visible radiation include: graphic arts films, proofing, printing plates, photoresists, and solder masks.
A large number of free-radical generating systems have been used as visible sensitizers for photopolymerizable compositions. Photoreducible dyes with various coinitiators have been widely studied. A useful discussion of dye sensitized photopolymerization can be found in "Dye Sensitized Photopolymerization" by D. F. Eaton in Adv. in Photochemistry, Vol. 13, D. H. Volman, G. S. Hammond, and K. Gollinick, eds., Wiley-Interscience, New York, 1986, pp. 427-487. However, many dye sensitized systems undergo dark reactions and are not shelf stable.
The preparation of holograms in photopolymerizable compositions, such as are disclosed in Haugh, U.S. Pat. No. 3,658,526; Keys, U.S. Pat. No. 4,942,102; Monroe, U.S. Pat. No. 4,942,112; Smothers, U.S. Pat. No. 4,959,284; and Trout, U.S. Pat. No. 4,963,471, require initiator systems which can be activated by visible radiation. However, it is desirable that, following imaging, the hologram not be colored by sensitizer, or its reaction products. The sensitizer must be colored to absorb actinic radiation and initiate photopolymerization, but must be photobleachable so that no colored products remain following imaging.
Photopolymerizable compositions containing 2,2',4,4',5,5'-hexaarylbisimidazoles, or HABI's, are well known. Sensitizers that extend the sensitivity of these compositions are disclosed in a number of patents. Baum and Henry, U.S. Pat. No. 3,652,275, discloses photopolymerizable compositions containing selected bis(p-dialkylaminobenzylidene)ketones as sensitizers of HABI initiator systems. Dueber, U.S. Pat. No. 4,162,162, discloses selected sensitizers derived from aryl ketones and p-dialkylaminoaldehydes. Smothers, U.S. Pat. No. 4,917,977, and Monroe, U.S. Pat. No. 4,987,230, also disclose visible sensitizers for HABI. However, these sensitizers can leave undesirable residual color in the hologram following imaging.
A need exists for initiator systems that are sensitive to visible radiation, especially in the green and red regions of the spectrum, but do not leave residual color in the image following imaging. These photobleachable initiator systems will be particularly useful in holography, pre-press color proofing, and in other applications in which an image with no residual color is desired.